The Boeing 767-300 was in cruise flight at flight level 350. The weather radar was on and no weather was depicted on the radar screen. The airplane encountered about 15 to 20 seconds of light chop which was followed by moderate turbulence lasting about 30 to 40 seconds. The seat belt sign was turned on, and the flight attendants were contacted to check on the status of the passengers. Initial report from the flight attendants revealed there were no injuries. A subsequent report from the chief purser, revealed three passengers were injured. The flight continued to the destination airport and landed without further incident. Probable Cause: Encounter with unforecasted weather (moderate turblence) resulting in injury to three passengers.